The present invention relates to a microporous foil and use thereof as a separator.
Modern devices rely on an energy source, such as batteries or rechargeable batteries, that enable the devices to be used in any location. Batteries have the disadvantage that they must be disposed of. Therefore, the use of rechargeable batteries (secondary batteries) that can be recharged repeatedly with the aid of chargers plugged into the mains is becoming more and more widespread. Nickel-cadmium (NiCd) rechargeable batteries, for example, have a service life of about 1000 recharging cycles if they area used correctly.
Batteries and rechargeable batteries always consist of two electrodes which are immersed in an electrolyte solution, and a separator, which separates the anode and the cathode from one another. The various types of rechargeable battery differ in the electrode material, the electrolyte, and the separator used. A battery separator has the task of keeping apart the cathode and the anode in batteries, or the negative and the positive electrode in rechargeable batteries. The separator must be a barrier that insulates the two electrodes from each other, to prevent internal short circuits. Yet at the same time the separator must be permeable for ions so that the electrochemical reactions can take place in the cell.
A battery separator must be thin, so that its internal resistance is as low as possible and high packing density can be achieved. This is the only way to achieve good performance data and high capacitances. It is also essential for the separators to soak up the electrolyte, and when the cells are full to ensure the exchange of gases. Whereas before fabrics or the like were used, nowadays most separators are made from microporous materials such as fleeces and membranes.
In lithium batteries, the occurrences of short circuits is a problem. Under thermal load, the battery separator in lithium ion batteries is prone to melt, resulting in a short circuit with disastrous consequences. Similar dangers exist if the lithium batteries are damaged mechanically or overcharged by chargers with faulty electronics.
In order to increase the safety of lithium ion batteries, shut-off membranes were developed. These special separators close their pores very rapidly at a given temperature, which is significantly lower than the melting point or ignition point of lithium. This largely prevents the catastrophic effects of a short circuit in lithium batteries.
At the same time, however, high mechanical strength is also desirable in separators, and this is lent to them by materials with high melting temperatures. For example, polypropylene membranes are advantageous because of their good resistance to perforation, but at about 164° C. the melting point of polypropylene is very close to the flame point of lithium (170° C.).
It is known in the related art to combine polypropylene membranes with other layers constructed from materials that have a lower melting point, for example polyethylene. Of course, such modifications of the separators must not impair the other properties such as porosity, nor hinder ion migration. However, the overall effect of including polyethylene layers on the permeability and mechanical strength of the separator is very negative. It is also difficult to get the polyethylene layers to adhere to polypropylene, and these layers can only be joined by laminating, or only selected polymers of both classes can be co-extruded.
There are essentially four different methods for manufacturing foils with high porosities known in the related art: filler methods, cold stretching, extraction methods, and β-crystallite methods. These methods differ fundamentally in the various mechanisms by which the pores are created.
For example, porous foils can be manufactured by adding very large quantities of filler materials. When they are stretched, the pores are created by the incompatibility between the filler materials and the polymer matrix. In many applications, the large quantities of as much as 40% by weight filler materials are associated with undesirable side effects. For example, the mechanical strength of such porous foils is reduced by the large content of filler materials despite stretching. Moreover, their pore size distribution is very wide, so that these porous foils are essentially unsuitable for use in lithium ion batteries.
In the “extraction methods”, the pores are created in principle by eluting a component from the polymer matrix with suitable solvent. In this context, a wide range of variants have been developed, and they differ in the types of additives and the suitable solvents that are used. Both organic and inorganic additives can be extracted. This extraction may be carried out as the last process step in the manufacture of the foil or it may be combined with a subsequent stretching step.
An older, method that has proven successful in practice relies on stretching the polymer matrix at very low temperatures (cold stretching). For this, the foil is first extruded in the normal way and then it is tempered for several hours to increase its crystalline content. In the following process step, it is cold stretched lengthwise at very low temperatures to create a large number of faults in the form of tiny microcracks. This prestretched, intentionally flawed foil is then stretched in the same direction again, with higher factors and at elevated temperatures, so that the flaws are enlarged to create pores that form a network-like structure. These foils combine high porosities with good mechanical strengths in the direction in which they are stretched, generally the lengthwise direction. However, their mechanical strength in the transverse direction remains unsatisfactory, which in turn means that their resistance to perforation is poor and they have a high tendency to splice in the lengthwise direction. The method is also generally expensive.
Another known method for producing porous foils is based on the addition of β-nucleating agents to polypropylene. In the presence of the β-nucleating agent, the polypropylene forms “β-crystallites” in high concentrations as the melt cools down. In the subsequent lengthwise stretching, the β-phase is converted into the alpha modification of the polypropylene. Since these different crystal forms vary in density, initially a large number of microscopic flaws are created here too, and they too are expanded to create pores by the stretching. The foils that are produced by this method have high porosities and good mechanical strengths both longitudinally and transversely and are extremely inexpensive. These foils will be referred to as β-porous foils in the following.
It is known that porous foils which are manufactured according to the extraction method may be provided with a shut-off function by the addition of a low-melting component. Since in this method orientation takes place first and the pores are created on the orientated foil afterwards by extraction, the low-melting component cannot hinder the formation of pores. Membranes with shut-off function are therefore often produced by this method.
Low-melting components may also be added to lend a shut-off function in the cold stretching method. The first stretching step must be carried out at very low temperatures anyway, in order to create the microcracks in the first place. The second, orientation step is generally performed in the same direction, usually MD, and may therefore also take place at a relatively low temperature, since the molecule chains are not re-orientated. The mechanical properties of these foils are deficient particularly in the transverse direction.
As an alternative, methods were developed in which various single-layer foils with different functions are first produced separately, then these are joined, that is to say laminated, to form a membrane with shut-off function. In this case, it is possible to optimise each layer individually with respect to its desired function without running the risk that that porosity of the membrane might be impaired by the shut-off function. Of course, these methods are very expensive and technically very involved.
Membranes consisting of β-porous foils have the drawback that until now they could only be provided with a corresponding shut-off function by laminating in this way. In order to create adequate porosities together with the desired mechanical strengths using β-crystallites and subsequent biaxial stretching, the foil must first be orientated longitudinally and then stretched transversely. Transverse stretching of a foil that has already been orientated longitudinally represents a de facto re-orientation of the polymer molecules and is contingent on significantly greater mobility of the polymer chains than is necessary for the first, lengthwise orientation of the unstretched polymers. Accordingly, transverse stretching of a polypropylene foil that has already been orientated longitudinally requires an elevated temperature, considerably higher than the desired shut-off temperature.
In the course of experiments relating to the present invention, it was therefore expected that the pores created by lengthwise and transverse stretching would be closed again by a low-melting component as early as the transverse stretching stage to such a degree that the porosity would be substantially limited. Lowering the transverse stretching temperature is subject to mechanical limits, since the longitudinally stretched polypropylene can only be stretched transversely at temperatures of at least 145° C., and generally undergoes transverse stretching at temperatures from 150 to 160° C. Consequently, there is no method known in the related art—except for lamination—by which β-porous foils can be provided with a shut-off function.